There are various mechanisms which may be responsible for the on-set of defects in blade roots, including high and low cycle fatigue. The presence of a defect may compromise the mechanical integrity of the blade, and as a result, it is important to have robust inspections methods to identify and quantify defects. This is commonly done by performing in-situ inspection by Non-Destructive Tests (NDT) methods that avoid disassembly of the blade row.
For a multi-stage steam turbine, due to the large number of scans required to obtain full coverage, in situ ultrasonic NDT inspections of last stage blades can last in the order of several days per turbine. One of the reasons for the length of time required is that echoes of the ultrasonic signals from the Phase Array probe from features of the foot platform interfere/hide signals from possible defects at relevant locations of the last stage blade. As a result, repeat scans are often performed. However, repeating a scan does not guarantee a previously hidden defect will be revealed as the assembled state of the blades significantly limits inspection angles and as a result is may not be possible, up until now, to avoid signal echoes from all blade features. If significant doubt remains, it may not be possible to avoid disassembly of the blade row and the use of another NDT technique, such as magnetic particle inspection, may be required.